Cardiac electrical activity. (A) Several ionic channels are present on the sarcolemma of cardiac myocytes: Na⁺, Ca²⁺, and K⁺ channels. Ca²⁺ channels are mainly located in transverses tubules when present. Specific ionic channels composition depends on the species and the region of the heart (e.g., atria versus ventricle). (B) Opening and closing of ionic channels are responsible for the action potential. (C) At the body level, surface ECG reflects the electrical activity in the entire heart. P wave is the atrial depolarization, QRS is the ventricular depolarization, and T wave is the ventricular repolarization. In some cases, T wave can be negative (dashed line).